1. Field of the Invention
This invention relates to a high sensitivity thermally developable light-sensitive material containing a light-sensitive silver halide catalyst prepared by a specific method.
2. Description of the Prior Art
Photographic methods using silver halides have hitherto been used most widely, which are superior in photographic properties such as sensitivity and gradation to electrophotographic methods and diazophotographic methods. However, the silver halide light-sensitive material used in such a photographic method is imagewise exposed, developed with a developing solution and then subjected to several processings such as stopping, fixing, water washing or stabilizing for the purpose of preventing the developed image from discoloration or fading and preventing the non-developed area (which will hereinafter be referred to as "background") from blackening. Therefore, this method has a disadvantage that these processings take much time and are laborsome and the handling of chemicals results in an injury to the human body and a contamination of workers' hands and clothes, and processing areas.
In the photographic method using a silver halide, therefore, it is very desirable to effect the processings in a dry condition without using solutions as well as to maintain the processed image stable.
To this end, many efforts have been made up to the present time. For example, a thermally developable light-sensitive material as disclosed in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,635,719 and 3,645,739 and Japanese Patent Publications 22185/1970 and 41865/1971 has been proposed. In this proposal, a light-sensitive element is used consisting of a silver salt of a long chain aliphatic carboxylic acid such as behenic acid, silver saccharin or silver benzotriazole and a catalytic amount of a silver halide.
The present invention relates to this thermally developable light-sensitive material. U.S. Pat. No. 3,457,075 describes a method of forming a catalytic amount of a light-sensitive silver halide used in the above described heat developable light-sensitive material, which comprises reacting an organic silver salt with a reactive and ionizable halide to form a light-sensitive silver halide. The description is that this light-sensitive silver halide catalyst is contacted catalytically with substantially all clusters of silver ions in the organic silver salt having silver ions associating in a cluster in its molecule, so that the catalyst is highly sensitive.
However, a light-sensitive silver halide catalyst having a sufficient sensitivity from a practical standpoint cannot be obtained by this method. For example, it is very difficult to apply the well-recognized sensitizing technique for ordinary silver halide emulsions (for wet development) such as sensitization by increasing the grain size of the silver halide or by using a chemical sensitizer with the light-sensitive silver halide catalyst prepared by this method due to the presence of, with the silver halide, a more unstable (reactive) organic silver salt than the silver halide.
If the silver halide catalyst can be prepared independently of the organic silver salt, it would be possible to apply known sensitizing techniques used with ordinary silver halide photographic emulsions. However, silver halide grains prepared in the presence of a protective colloid such as gelatin in order to prevent the silver halide from aggregation are not suitable for use as a light-sensitive silver halide catalyst for a thermally developable light-sensitive material, because the protective colloid of the silver halide adsorbs strongly on the silver halide, resulting in an inadequate contact of the silver halide and organic silver salt. However, a silver halide prepared in the absence of a protective colloid is also not suitable for the light-sensitive silver halide catalyst, because the silver halide in the absence of the protective colloid tends to aggregate and thus is not brought into catalytic contact the organic silver salt. This is apparent from U.S. Pat. Nos. 3,152,904 and 3,457,075. According to U.S. Pat. No. 3,457,075, for example, the thermally developable light-sensitive material prepared by mixing only silver chloride and silver saccharin as in Example 2 of U.S. Pat. No. 3,152,904 has a very low sensitivity since the silver chloride and silver saccharin are not brought into catalytic contact. In fact, the sensitivity is low.